The fast neutron contamination associated with photon beams of high-energy medical accelerators is a matter of concern to the health community because this radiation may contribute to a significant whole-body dosage thereof to a patient undergoing therapy.
Recently there has been a trend toward the use of higher energy electron linear accelerators as X-ray sources for radiotherapy. When operated at 25 MeV in the X-ray mode, a significant fraction of the photons are capable of initiating n-.gamma. reactions. Neutron dose rates of approximately 0.5% of the X-ray dose rate at a distance of 1 m from the target of a 25 MeV medical accelerator have been reported (1). Although the neutron dose is small, the rem dose may be biologically significant when considering the effect on the eyes (1) and other organs of the body.
In order to quantify the neutron contamination for evaluation of its biological effects, it is necessary to determine its energy spectrum. There are three major problems associated with the determination of neutron spectra from medical linacs: (a) the radiation field is a mixture of a high-intensity gamma-ray field and a low intensity neutron field; (b) the machine operates in a pulsed mode, causing high flux rates for very short periods, typically less than 2.mu.s; and (c) the neutron spectrum must be determined from the pulse-height distribution of the secondary particles (protons). Once the energy spectrum of the neutrons has been determined, the resultant tissue kerma rate may be calculated.
In the past, neutron spectrometry required bulky nuclear instrumentation and a minicomputer to determine the neutron energy spectrum. There is a significant need for a portable instrument capable of detecting, computing and displaying neutron spectra and kerma rate in a mixed field of radiation, and which is physically suitable for utilization at a wide range of locations or sites where such radiation is suspected to be present.
A preliminary search of the prior patented art revealed the following prior U.S. patents of interest in connection with the present invention:
Berlman et al, U.S. Pat. No. 2,795,703 PA0 Scherbatskoy, U.S. Pat. No. 2,830,189 PA0 Love et al, U.S. Pat. No. 3,129,329 PA0 Sleege, U.S. Pat. No. 3,519,822 PA0 Kawashima, U.S. Pat. No. 3,898,466 PA0 Kamburov et al, U.S. Pat. No. 3,056,725
Also of interest are the following publications, to which reference is made in the course of the description of the present invention:
1. R. M. Wilenzich, P. R. Almond, G. D. Oliver, Jr., and C. D. DeAlmeida, "Measurements of Fast Neutrons Produced by High-Energy X-Ray Beams of Medical Electron Accelerators", Phy. Med. Biol. 18 (1973) 396.
2. M. L. Roush, M. A. Wilson and W. F. Hornyak, "Pulse Shape Discrimination", Nucl. Instr. and Meth. 31 (1964) 112.
3. D. W. Glasgow, D. E. Velkley, J. D. Brandenberger, and M. T. McEllistrem, "Pulse-Shape Discrimination for Wide Dynamic Range Neutron Scattering Experiments", Nucl. Inst. and Meth. 114 (1974) 535.
4. I. J. Taylor and J. Kalyna, "A High Speed Pulse Shape Discriminator", Nucl. Inst. and Meth. 88 (1970) 267.
5. Mike Wiles and Andre Felix, MCM6830L7 MIKBUG/MINIBUG ROM, Engineering Note 100, Motorola, Inc. 1975.
6. H. W. Broek and C. E. Anderson, "The Stilbene Scintillation Crystal as a Spectrometer for Continuous Fast-Neutron Spectra", Rev. Sci. Instrum. 10 (1960) 1063.
7. D. W. Jones and M. Elaine Toms, "A Neutron Spectrometer Using Organic Scintillators", NRL Report 7324, Washington, D.C. 1971.
8. J. J. Ritts, M. Solomito and P. N. Stevens, "Calculation of Neutron Fluence-to-Kerma Factors for the Human Body", Nucl. Appl. and Tech. 1 (1969) 89.
9. Charles J. Daniels, "A Portable Scintillation Counter with Pulse-Shape Discrimination for Measurement of Fast Neutron Spectra and Dose in a Mixed n-.gamma. Field", M. S. Thesis, Physics Department, University of Maryland, Mar. 29, 1977.
10. Jeffrey L. Silberberg, "Design of a Microcomputer Processing Subsystem for a Portable Neutron Spectrometer/Kerma-Rate Meter", M. S. Thesis, Electrical Engineering Department, University of Maryland, May 11, 1977.
11. Jeffrey L. Silberberg, "A Microprocessor System for a Portable Neutron Spectrometer/Kerma-Rate Meter", IEEE Transactions on Nuclear Science, Vol. NS-24, No. 1, Feb. 1977.